In permanent magnet (PM) type machines, the presence of the magnet generates a magnetic flux, even when lacking a stator current. This flux typically results in a back electromotive force (EMF) that is proportional to the motor speed and that may increase beyond a direct current (DC) bus voltage. For example, the phase voltage of a PM machine increases as the velocity of the machine is increased. Above a predetermined velocity, the peak line-to-line voltage of the PM machine becomes greater than the bus voltage. To retain current control of the PM machine, the back EMF is generally reduced using field-weakening. A variety of field-weakening techniques are available, and the selection of a particular field-weakening technique may be based on the characteristics of the PM machine (e.g., a surface mount PM type machine, an interior PM type machine, or the like) and/or the requirements of the overall system.
In PM machines, a demagnetizing current is typically applied to reduce the magnet or total flux of the PM machine. Currently, d-axis current control techniques have been used to field-weaken the back EMF in strong magnet flux PM machines. For example, a negative d-axis current may be applied to produce a demagnetizing flux component that reduces the d-axis flux and the back EMF. These d-axis current control techniques have limited success with weak flux PM machines due to the weak influence of the d-axis current on the machine voltage. For example, in the non-linear overmodulation region of operation, the low influence of the d-axis current on the total machine flux may impair field-weakened operation of a weak flux PM machine. Additionally, under a large demagnetizing current, the d-axis flux may reverse polarity for some PM machines. In this case, increasing the negative d-axis current can cause instability by increasing the total voltage magnitude instead of decreasing the same. Hence, d-axis current control may be inadequate for some PM machines that operate at a high velocity with a large demagnetizing current.
In one field-weakening technique, look-up tables are utilized to generate initial d- and q-axis current commands in a feed-forward manner. For example, the characteristics of the PM machine may be measured and used to develop a model that is used to determine efficiency-optimized control parameters. These control parameters are typically implemented as look-up tables for efficiency-optimized control of the machine. The control parameters may also be determined within the voltage and current limits. During an ideal operation, a feed-forward control using these control parameters is generally sufficient to provide stable control of the PM machine under steady state conditions.
In an ideal case, these feed-forward commands, when implemented and applied to an AC motor operating with load, will typically produce the desired stator voltage. At higher speeds and/or torques, the stator voltage can be large, and appropriate selection of the look-up tables may keep the stator voltage within the inverter voltage limits. To retain current control at high speeds, when the available voltage is limited, additional assistance may be needed especially during transient operations or in the event of a mismatch between the actual machine parameters and the measured parameters. Due to the presence of non-idealities, such as parameter variation (e.g., manufacturing tolerances, temperature, aging, etc.), the stator voltage tends to deviate from the desired value. Additionally, during dynamic operation, additional stator voltage may be desirable, which may not be provided by the steady-state commands of the look-up tables. A field-weakening voltage loop is typically used to correct the errors between the model and the actual machine parameters for a stable machine operation under load.
Accordingly, it is desirable to provide systems and methods for controlling PM machines that accomodate a variety of load conditions. Additionally, it is desirable to provide systems and methods for controlling PM machines during operation under no load or low load conditions that also provide fast dynamics and steady-state stability. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.